The invention relates to that aspect of ophthalmic surgery which is concerned with operations upon the external surface of the cornea.
Operations of the character indicated include corneal transplant and keratotomies; such operations have traditionally required skilled manipulation of a cutting instrument. But, however keen the cutting edge, the mere entry of the edge into the surface of the cornea necessarily means a wedge-like lateral pressure against body cells displaced by the entry, on both sides of the entry. Such lateral pressure is damaging to several layers of cells on both sides of the entry, to the extent impairing the ability of the wound to heal, and resulting in the formation of scar tissue.
My original patent application Ser. No. 552,983, filed Nov. 17, 1983, includes a background discussion of the effects of various available wavelengths of laser radiation in ophthalmic surgery and, in particular, surgery performed on the anterior surface of the cornea. It is explained that radiation at ultraviolet wavelengths is desirable by reason of its high photon energy. This energy is greatly effective on impact with tissue, in that molecules of tissue are decomposed on photon impact, resulting in tissue ablation by photodecomposition. Molecules at the irradiated surface are broken into smaller volatile fragments without heating the remaining substrate; the mechanism of the ablation is photochemical, i.e., the direct breakdown of the intra-molecular bonds. Photothermal and/or photocoagulation effects are neither characteristic nor observable in ablations at ultraviolet wavelengths, and cell damage adjacent the ablation is insignificant.
Said related-case applications deal with various concepts whereby laser radiation at ultraviolet wavelengths of 200-nm or less are controlled in delivery of laser radiation to the visually used area of the anterior surface of the cornea so as to penetrate the stroma and achieve a predeterminable volumetric removal of corneal tissue, thereby so correctively changing the profile of the anterior surface as to reduce a myopia, or a hyperopia, or an astigmatic abnormality which existed prior to such laser surgery.
Said related-case applications were concerned primarily with the methods and means of achieving desired corneal sculpture through controlled delivery of ultraviolet laser radiation. The disclosures of these applications were addressed to the ophthalmic surgeons who presumably are skilled in traditional procedures; but, although the laser-sculpting procedures I disclosed were contrary to current professional practice and beliefs, in the sense that I called for ablation depths which necessarily involved traversing Bowman's membrane in order to penetrate the stroma, I have been surprised that those skilled in laser technology who would attempt to experimentally apply my disclosures to their own research, have been unduly preoccupied with the epithelium, namely, the thin regrowable layer which nature provides for protection of the anterior surface of the cornea. I have found that such preoccupation with the epithelium can not only produce an undesirable result but can also be a reason for unpredictability of a desired result.
In my copending application Ser. No. 059,617, filed June 8, 1987, note is taken of the fact that the density of Bowman's membrane tissue, for which thickness is in the range from 10 to 15 microns, exceeds the density of underlying stroma, and a technique is described for achieving ablated recurvature of the anterior surface of the cornea by first making a uniform-depth penetration through Bowman's membrane, so that laser-sculpting action can proceed essentially only within stroma tissue. This procedure is effective, whatever the prescribed diopter change (i.e., for a relatively large range of diopter change), but of course it means that total cornea-penetration depth must be the sum of the depth required to fully penetrate Bowman's layer, plus the stroma-penetration depth required for the particular prescription diopter change; for example, for a 5-mm diameter circular area which is to receive a two-diopter curvature change, the maximum penetration depth required for the recurvature is about 20 microns, and this must be added to the 15 microns required to assure uniform penetration through Bowman's membrane.
The present invention concerns itself with prescription diopter changes that require relatively small ablative penetration depths, as for example when the prescription area to be sculpted is of diameter less than 5-mm, or when only a one or two-diopter change is specified for a 5-mm diameter area.